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PDF LT8611 Data sheet ( Hoja de datos )
| Número de pieza | LT8611 | |
| Descripción | 2.5A Synchronous Step-Down Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
n Rail-to-Rail Current Sense Amplifier with Monitor
n Wide Input Voltage Range: 3.4V to 42V
n Ultralow Quiescent Current Burst Mode® Operation:
2.5μA IQ Regulating 12VIN to 3.3VOUT
Output Ripple < 10mVP-P
n High Efficiency Synchronous Operation:
96% Efficiency at 1A, 5VOUT from 12VIN
94% Efficiency at 1A, 3.3VOUT from 12VIN
n Fast Minimum Switch-On Time: 50ns
n Low Dropout Under All Conditions: 200mV at 1A
n Allows Use Of Small Inductors
n Low EMI
n Adjustable and Synchronizable: 200kHz to 2.2MHz
n Current Mode Operation
n Accurate 1V Enable Pin Threshold
n Internal Compensation
n Output Soft-Start and Tracking
n Small Thermally Enhanced 3mm × 5mm 24-Lead
QFN Package
APPLICATIONS
n Automotive and Industrial Supplies
n General Purpose Step-Down
n CCCV Power Supplies
Electrical Specifications Subject to Change
LT8611
42V, 2.5A Synchronous
Step-Down Regulator with
Current Sense and 2.5µA
Quiescent Current
DESCRIPTION
The LT®8611 is a compact, high efficiency, high speed
synchronous monolithic step-down switching regulator
that consumes only 2.5μA of quiescent current. Top and
bottom power switches are included with all necessary
circuitry to minimize the need for external components.
The built-in current sense amplifier with monitor and
control pins allows accurate input or output current
regulation and limiting. Low ripple Burst Mode operation
enables high efficiency down to very low output currents
while keeping the output ripple below 10mVP-P. A SYNC
pin allows synchronization to an external clock. Internal
compensation with peak current mode topology allows
the use of small inductors and results in fast transient
response and good loop stability. The EN/UV pin has an
accurate 1V threshold and can be used to program VIN
undervoltage lockout or to shut down the LT8611 reduc-
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ing the input supply current to 1μA. A capacitor on the
TR/SS pin programs the output voltage ramp rate during
start-up. The PG flag signals when VOUT is within ±9% of
the programmed output voltage as well as fault conditions.
The LT8611 is available in a small 24-lead 3mm × 5mm
QFN package with exposed pad for low thermal resistance.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
TYPICAL APPLICATION
5V Step-Down Converter with 2.5A Output Current Limit
VIN
5.5V TO 42V
4.7μF ON OFF
VIN
EN/UV
BST
SYNC
LT8611
SW
IMON
ICTRL
ISP
ISN
BIAS
INTVCC
PG
TR/SS
0.1μF
RT
FB
PGND GND
1μF 52.3k
0.1μF
4.7μH
0.02Ω
1μF
VOUT
5V
2.5A
10pF
1M
243k
47μF
fSW = 800kHz
8611 TA01a
12VIN to 5VOUT Efficiency
100
95
90
85
80
75
70
65
60
55
50
0
fSW = 700kHz
VIN = 12V
VIN = 24V
0.5 1 1.5 2 2.5
LOAD CURRENT (A)
8611 TA01b
8611p
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LT8611
No Load Supply Current
5.0
4.5
VOUT = 3.3V
IN REGULATION
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0 5 10 15 20 25 30 35 40 45
INPUT VOLTAGE (V)
8611 G10
Top FET Current Limit
5.0
4.5 30% DC
4.0
70% DC
3.5
3.0
2.5
–55 –25 5
35 65
TEMPERATURE (°C)
95 125
8611 G14
Switch Drop
450
400
350
300
250
TOP SW
200
BOT SW
150
100
50
0
0 0.5 1 1.5 2 2.5 3
SWITCH CURRENT (A)
8611 G41
No Load Supply Current
25
VOUT = 3.3V
VIN = 12V
20 IN REGULATION
15
10
5
0
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
8611 G11
Bottom FET Current Limit
3.6
3.4
3.2
3.0
2.8
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2.6
2.4
–55 –25 5
35 65
TEMPERATURE (°C)
95 125
8611 G15
Minimum On-Time
80
ILOAD = 1A, VSYNC = 0V
75 ILOAD = 1A, VSYNC = 3V
70
ILOAD = 2.5A, VSYNC = 0V
ILOAD = 2.5A, VSYNC = 3V
65
60
55
50
45
40
35
30
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
8611 G17
Top FET Current Limit vs Duty Cycle
4.4
VOUT = 3.3V
4.2
4.0
3.8
3.6
3.4
3.2
3.0
5
10 15 20 25 30 35 40 45
INPUT VOLTAGE (V)
8611 G12
Switch Drop
250
SWITCH CURRENT = 1A
200
150
TOP SW
100
BOT SW
50
0
–55 –25
5 35 65 95
TEMPERATURE (°C)
125 155
8611 G40
Minimum Off-Time
100
VIN = 3.3V
95 ILOAD = 0.5A
90
85
80
75
70
65
60
–50 –25
5 35 65 95
TEMPERATURE (°C)
125 155
8611 G18
8611p
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LT8611
OPERATION
The LT8611 is a monolithic, constant frequency, current
mode step-down DC/DC converter. An oscillator, with
frequency set using a resistor on the RT pin, turns on
the internal top power switch at the beginning of each
clock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which
the top switch turns off is controlled by the voltage on
the internal VC node. The error amplifier servos the VC
node by comparing the voltage on the VFB pin with an
internal 0.97V reference. When the load current increases
it causes a reduction in the feedback voltage relative to
the reference leading the error amplifier to raise the VC
voltage until the average inductor current matches the new
load current. When the top power switch turns off, the
synchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload
conditions result in more than 3.3A flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
The LT8611 includes a current control and monitoring
loop using the ISN, ISP, IMON and ICTRL pins. The ISP/
ISN pins monitor the voltage across an external sense
resistor such that the VISP-VISN does not exceed 50mV
by limiting the peak inductor current controlled by the VC
node. The current sense amplifier inputs (ISP/ISN) are rail-
to-rail such that input, output, or other system currents
may be monitored and regulated. The IMON pin outputs
a ground-referenced voltage equal to 20 times the voltage
between the ISP-ISN pins for monitoring system currents.
The ICTRL pin can be used to override the internal 50mV
limit between the ISP, ISN pin to a lower set point for the
current control loop.
If the EN/UV pin is low, the LT8611 is shut down and
draws 1μA from the input. When the EN/UV pin is above
1V, the switching regulator will become active.
To optimize efficiency at light loads, the LT8611 operates
in Burst Mode operation in light load situations. Between
bursts, all circuitry associated with controlling the output
switch is shut down, reducing the input supply current to
1.7μA. In a typical application, 2.5μA will be consumed
from the input supply when regulating with no load. The
SYNC pin is tied low to use Burst Mode operation and can
be tied to a logic high to use pulse-skipping mode. If a
clock is applied to the SYNC pin the part will synchronize to
an external clock frequency and operate in pulse-skipping
mode. While in pulse-skipping mode the oscillator operates
continuously and positive SW transitions are aligned to
the clock. During light loads, switch pulses are skipped
to regulate the output and the quiescent current will be
several hundred μA.
To improve efficiency across all loads, supply current to
internal circuitry can be sourced from the BIAS pin when
biased at 3.3V or above. Else, the internal circuitry will draw
current from VIN. The BIAS pin should be connected to
VOUT if the LT8611 output is programmed at 3.3V or above.
Comparators monitoring the FB pin voltage will pull the
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PG pin low if the output voltage varies more than ±9%
(typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8611’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the inductor current when the
output voltage is lower than the programmed value which
occurs during start-up or overcurrent conditions. When
a clock is applied to the SYNC pin or the SYNC pin is
held DC high, the frequency foldback is disabled and the
switching frequency will slow down only during overcur-
rent conditions.
8611p
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11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT8611.PDF ] | |
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